Refrigeration units for frozen food transport trucks, and other refrigeration applications comprise generally a metal holdover plate having serpentine tubing coiled throughout the plate and a heat storage material disposed within the holdover plate and surrounding the tubing. The refrigerant, e.g. freon, NH.sub.3, etc., flows through the tubing. The heat storage material used is generally a eutectic mixture. A eutectic mixture is a precise mixture of two or more substances which in combination have a single melting/freezing point, which will be referred to herein as a "transition temperature". Technically, a eutectic mixture exhibits the lowest transition temperature of solidification for any mixture of specified constituents. A near-eutectic mixture is one which has a transition temperature slightly above and a melting temperature profile somewhat broader than that of the eutectic mixture of the same components.
Holdover plates and similar refrigeration systems operate by freezing the heat storage material at a temperature specific for the particular material so that the material maintains the system at or near its transition temperature. The capacity of the heat storage material to maintain the system at a specified temperature is a function, in part, of the heat of fusion for the heat storage material, with materials having a higher heat of fusion being more capable of maintaining the specified temperature of the system for a longer period of time.
Thus, a heat storage material for a particular system is selected having a transition temperature that is at the temperature which is desired to maintain the goods, products or objects to be refrigerated.
Another factor in the ability of a holdover plate to maintain a system at a predetermined temperature is the effectiveness of the heat transfer within the holdover plate material and from the holdover plate material to the refrigerated space. Certain metals such as copper, aluminum and brass have greater heat transfer capability than other metals, such as steel, and would be more desirable materials for use in the construction of the holdover plates. However, prior art holdover plate manufacturers must take great care in their selection of the material used to manufacture the plates because there is a significant problem of plate corrosion. This corrosion problem is primarily based on the fact that the most commonly used eutectic materials in the prior art are brine solutions generally comprising a metal halide salt such as sodium chloride dissolved in water.
Although brine solutions have relatively high heats of fusion compared with other prior art heat storage materials such as glycol-water, alcohol-water, ethylene-water and propylene glycol-water, the major drawback of brine solutions is that they are extremely corrosive, and require special construction techniques to eliminate electrolytic corrosion.
For example, one prior art holdover plate is made of steel coated on its entire outer surface with hot molded zinc. All of the fittings, shell, tubing, fins, spacers and welding rods are formed of the same type of steel alloy. After forming the holdover tank, it is filled with a brine solution and then sealed under a vacuum to remove air bubbles from the brine solution and provide a small amount of space for expansion of the heat storage material contained therein. As is well known in the art, the extra care required to manufacture such holdover plates makes such plates very expensive, although less so than utilizing all stainless steel parts in the holdover plates. It is primarily the corrosive nature of the brine solutions used in prior art holdover plates which requires the special care.
Two prior art patents described the use of ethylene diamine as a heat storage material, namely Hill, et al U.S. Pat. Nos. 2,327,041 and 2,333,862. However, both patents only disclose the use of ethylene diamine for temperatures in the range of 32.degree. F. to 50.degree. F. which is substantially above temperatures considered effective for transporting frozen foods.
The disadvantages of the prior art heat storage materials discussed above have been overcome in the present invention, which is described below.